Foaming synthetic resin compositions stabilized with N-(higher alkyl) C.sub. -C8 amides

ABSTRACT

Substantially closed-cell aliphatic olefin polymer foams having improved dimensional stability are provided by the inclusion of from 0.1 to 10 weight percent, based on olefin polymer, of an N-higher aliphatic hydrocarbyl substituted amide of a C1-C8 aliphatic carboxylic acid.

BACKGROUND OF THE INVENTION

The present invention relates to the foaming or expansion of syntheticresin compositions and, more particularly, to foaming processes,foamable compositions and foamed articles involving an aliphatic olefinpolymer, a blowing agent and from 0.1 to 10 percent by weight, based onsaid aliphatic olefin polymer of at least one compound effective tosuppress the escape of the blowing agent gas from the expanded foam,with a resultant improvement in the gas efficiency of the blowing agenttogether with a substantial reduction in shrinkage of the expanded foam.

Generally, thermoplastic synthetic resin foam is produced by adding ablowing agent to the resin and then heating the resultant resin mixtureto gasify the blowing agent for expanding the resin. Improvement in theefficiency of such blowing agents is significant for commercial-scaleproduction of foams.

Among thermoplastic resin, aliphatic olefin polymer resins areparticularly difficult to expand directly in a commercial-scale process.This has been attributed to the the high temperature dependence of theirthermo-rheological viscoelasticity in the molten state and the gaspermeability of their foam cell membranes. Further complications arisefrom the latent heats of thermal expansion and crystallization which mayoccur during the foaming process. Commonly, especially for expansionratios of 5 or more, aliphatic olefin polymers are modified prior toexpansion, by crosslinking or blending with other resin or resins.

As an alternative approach, Japanese Patent Publication No.35(1960)-4,341 discloses extrusion-foaming of a non-crosslinked olefinpolymer resin using such a relatively expensive blowing agent as1,2-dichlorotetrafluoroethane. However, the resulting foam product oftenundergoes a gradual shrinkage and a density change and has a poorsurface appearance with many wrinkles. Improved quality and dimensionalcontrol of the foams produced by such process are desirable.

More recently, a variety of chemical additives have been discoveredwhich improve the quality and dimensional stability of olefin polymerfoams without the necessity of crosslinking and/or resin blendingtechniques while permitting the satisfactory utilization of lessexpensive volatile organic blowing agents in place of1,2-dichlorotetrafluoroethane. More specifically, such additives includesaturated higher fatty acid amides, saturated higher fatty acid aminesand complete esters of saturated fatty acids as disclosed in Watanabe etal., U.S. Pat. No. 4,214,054, partial esters of long chain fatty acidsas disclosed in Cronin, U.S. Pat. No. 3,644,230 and Ehrenfreund, U.S.Pat. No. 3,755,208; partial and complete esters of aliphatic fatty acidsas disclosed in Komori, U.S. Pat. No. 4,217,319; and the relatedtechnology of Japanese Kokai Nos. 53-102,971, 54-34,374 and 54-39,476.

The present invention provides another group of chemical additives whichimprove the dimensional stability of aliphatic olefin polymer foams.More specifically, the invention provides a foamable olefin polymercomposition comprising a normally solid aliphatic olefin polymer whichis easily expanded using commonly available inexpensive blowing agentsto give a substantially closed-cell foam product exhibiting reducedshrinkage and improved dimensional stability and surface smoothness.

SUMMARY OF THE INVENTION

In accordance with the present invention such a foamable olefin polymercomposition comprises a major proportion by weight of a normally solidaliphatic olefin polymer, a volatile organic blowing agent and from 0.1to 10 weight percent, based upon such olefin polymer, of at least oneN-(higher aliphatic hydrocarbyl) substituted lower aliphatic hydrocarbylamide of formula I: ##STR1## wherein R₁ is an aliphatic hydrocarbongroup having 10-24 carbon atoms, R₂ is a hydrogen atom, an aliphatichydrocarbon group having 1-24 carbon atoms, or oxyalkylene orpolyoxyalkylene group represented by --R₄ --O--_(n) A wherein R₄ is aresidual group of an aliphatic polyhydric alcohol having 1-5 carbonatoms, A is a hydrogen atom or a residual group of an aliphaticcarboxylic acid having 1-24 carbon atoms and n is an integer rangingfrom 1 to 20; and R₃ is a hydrogen atom or an aliphatic hydrocarbongroup having 1-7 carbon atoms.

These foamable aliphatic olefin polymer compositions are particularlyadvantageous for use with non-crosslinked aliphatic olefin polymers andwith relatively inexpensive volatile organic blowing agents and providegood quality substantially closed-cell aliphatic olefin polymer foamshaving improved dimensional stability. The availability of differenttypes of additives and stabilizers provides greater operatingflexibility to foam manufacturers.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing foam volume in percent based upon initial foamvolume as a function of time for foam formed from Formulation 1 inExample 1 and from Formulation 1 in Reference 1 of the accompanyingworking examples.

FIG. 2 is a graph showing foam weight in percent based upon initial foamweight as a function of time for foam formed from the same Formulations.

DETAILED DESCRIPTION OF THE INVENTION

Suitable stabilizers herein are amides of formula I: ##STR2## wherein R₁is an aliphatic hydrocarbon group having 10 to 24 carbon atoms such asdecyl, dodecyl, palmityl, stearyl, oleyl, docosyl, docosenyl, andtetracosyl groups. R₂ represents a hydrogen atom; an aliphatichydrocarbon group having 1 to 24 carbon atoms such as methyl, ethyl,propyl, butyl, hexyl, octyl, decyl, dodecyl, palmityl, stearyl andtetracosyl groups; or an oxyalkylene or polyoxyalkylene grouprepresented by --R₄ --O--_(n) A wherein R₄ is a residual group of analiphatic polyhydric alcohol having 1 to 5 carbon atoms such asmethylene, ethylene, trimethylene, and tetramethylene (including itsisomer) groups, including residual groups of polydihydric alcohols suchas polyethylene glycol, polypropylene glycol and polytetramethyleneglycol as well as mixtures thereof, with n being an integer ranging from1 to 20 and A being a hydrogen atom or a residual group of an aliphaticcarboxylic acid having 1 to 24 carbon atoms such as formic acid, aceticacid, butyric acid, isobutyric acid, caproic acid, caprylic acid, capricacid, lauric acid, palmitic acid, stearic acid, oleic acid, behenic acidand erucic acid. R₃ represents a hydrogen atom or an aliphatichydrocarbon group having 1 to 7 carbon atoms such as methyl, ethyl,propyl, butyl, isobutyl, hexyl and heptyl groups.

The preferred combination of R₁, R₂ and R₃ cannot be statedunconditionally because it varies depending on the type of the aliphaticolefin polymer employed and the properties of the blowing agent used aswell as on the density of the intended foam. However, it is generallypreferable that R₁ is an aliphatic hydrocarbon group having 12 to 22carbon atoms, that R₂ is a hydrogen atom or a polyoxyalkylene grouprepresented by --R₄ --O--_(n) A described hereinabove, and that R₃ is ahydrogen atom or an aliphatic hydrocarbon group having 1 to 4 carbonatoms.

The effects of the present invention are not satisfactorily achieved ifR₁ has 9 or less carbon atoms or if R₃ has 8 or more carbon atomsperhaps because such compounds do not provide a necessary combination ofcohesiveness to and solubility in the aliphatic olefin polymers employedherein.

Typical examples of the compounds represented by formula I are N-decylacetamide, N-palmityl acetamide, N-stearyl acetamide and N-docosylacetamide, including polyoxyethylene derivatives thereof as well asacetate, caprinate, palmitate, stearate, oleate, behenate and erucatederivatives of these polyoxyethylene derivatives. These compounds mayalso be used as mixtures thereof.

According to the present invention, the foaming resin composition shouldcontain at least 0.1 percent by weight, based on the aliphatic olefinpolymer, of at least one compound represented by the formula I, with thetotal content thereof falling in the range of 0.1 to 10 percent byweight when two or more such compounds are used. The amount of addedcompound or compounds may be selected within this range depending on thetypes of polymer and blowing agent used as well as on the shape,physical and mechanical properties of the intended foam. Generally, itis preferable that the total amount be in the range of 0.3 to 7 percentby weight. If the content of formula I compound(s) is smaller than 0.1percent by weight, the resultant foam will have significantly inferiorproperties. On the other hand, using more than 10 percent by weightgenerally gives no further improvement and may show undesirableplasticizing action.

The aliphatic olefin polymers herein referred to are normally solidpolymers predominantly composed of olefins, including low-densitypolyethylene, medium-density polyethylene, high-density polyethylene,isotactic polypropylene and poly-1-butene, and such copolymers ofethylene or propylene and other monomers copolymerizable therewith aspropylene-(1-octene)-ethylene copolymer, ethylene-propylene copolymer,ethylene-vinyl acetate copolymer, ethylene-acrylic acid copolymer,ethylene-ethyl acrylate copolymer and ethylene-vinyl chloride copolymeras well as zinc, sodium, calcium and magnesium salts of ethylene-acrylicacid copolymer. These olefin polymers may be used individually or asmixtures of two or more.

For mixing the formula I compound(s) of the present invention with thealiphatic olefin polymers to be employed such components may be admixedand kneaded by any suitable means such as single-screw extruder,twin-screw extruder, compounding rolls, and Banbury mixer. In any case,a preliminary blend of the formula I compound(s) and the olefin polymermay be prepared as a dryblend or masterbatch, or the formula I compoundmay be mixed with the olefin polymer in molten or heat plastified formjust before expansion.

The expandable olefin polymer composition of the present invention maybe expanded into a foam by conventional methods. For example,conventional extrusion foaming techniques can be employed in which anadmixture of the olefin polymer and the formula I compound(s) of thepresent invention is continuously heated and/or worked to be melted orheat plastified and a volatile organic blowing agent is added thereto atan elevated temperature under high pressure. Then, the resultant moltenor heat plastified mixture is extruded into a lower-pressure zone to beexpanded into a foam. Alternately, a batch method may be used in whichthe volatile organic blowing agent is added at elevated temperatureunder high pressure to the molten olefin polymer composition and thenpressure is removed from the molten mixture system. Also, the olefinpolymer/formula I compound composition can be crosslinked with electronbeams or a chemical crosslinking agent if desired before expansion.However, the present invention is particularly effective when applied toextrusion-expansion of olefin polymer compositions and especially whensuch compositions are to be expanded to five or more times theiroriginal, unexpanded volume.

Any conventional volatile organic blowing agents may be used as desired.However, especially preferable for the present invention are volatileorganic blowing agents that have boiling points lower than the meltingpoint of the aliphatic olefin polymer employed. Typical examples of suchpreferrable blowing agents include propane, butane, pentane, pentene,hexane, hexene, heptane and octane. Also usable as the blowing agentsare such halogenated hydrocarbons meeting the foregoing limitation onthe boiling points as methylene chloride, trichlorofluoromethane,dichlorofluoromethane, chlorodifluoromethane, chlorotrifluoromethane,dichlorodifluoromethane, 1,1-dichloroethane,1-chloro-1,1-difluoroethane, 1,8-dichlorotetrafluoroethane,chloropentafluoroethane and the like, including mixtures thereof. Inaddition to such volatile organic blowing agents, known chemical blowingagents such as azodicarbonamide, azobisisobutyronitrile,dinitrosopentamethylenetetramine, p-toluenesulfonyl hydrazide and thelike can also be optionally employed in conjunction with such volatileorganic blowing agents.

The expandable or foamable olefin polymer composition according to thepresent invention may contain minor amounts of lubricating andnucleating agents such as zinc stearate or similar metal soaps andfinely-ground calcium silicate or the like inorganic materials. Also,the resin composition of the present invention may contain anultraviolet light absorber, antistatic agent, stabilizer, colorantand/or lubricating agent other than the foregoing in a total amount notexceeding 5 percent by weight.

The foamable aliphatic olefin polymer compositions according to thepresent invention may be expanded into almost any shapes includingsheets, blocks, rods and pipes, or it may be used for electric wire andcable coverings or sheathings or as a variety of other formed articles.

The present invention is further illustrated by the following examplesand accompanying comparative or reference experiments. The resultingfoam products were evaluated in terms of the post-expansion shrinkage(shrinkage as a function of time after expansion) and the surfacecondition of the foams after stabilization of their dimensions. Allparts and percentages are given on a weight basis unless otherwisespecified.

Post-expansion shrinkage (in percent of initial expanded volume) wasdetermined using round foam rod test specimens (about 25 mm across and50 cm long) obtained by extrusion foaming of the indicated olefinpolymer compositions. Immediately after expansion and n days thereafter,the diameter and length of each specimen were measured with a verniercaliper (JIS grade 1) and steel rule (JIS grade 1) to determine itsvolume. The post-expansion shrinkage was then calculated from theformula: ##EQU1##

The surface condition of the test specimens was evaluated using thefollowing three relative ranks:

    ______________________________________                                        Evaluation Symbols                                                                           Surface Conditions                                             ______________________________________                                        G - Good       Surface having excellent smooth-                                              ness almost without wrinkles                                   F - Fair       Surface having visible wrinkles,                                              but commercially acceptable in                                                general                                                        P - Poor       Surface having many wrinkles                                                  significantly degrading the com-                                              mercial value of the foam                                      ______________________________________                                    

In industrial practice it is preferable that the post-expansionshrinkage of a foam is minimized. Generally, the initial shrinkage(shrinkage in one day after expansion) should be about 15 percent orless. If the initial shrinkage exceeds this level, a much longer time(about one month) is required for the foam to substantially recover itsoriginal dimensions and the foam will generally have an unacceptablesurface condition. Further, it is desired, that the difference betweenthe initial shrinkage and the shrinkage measured 10 days after expansionbe not more than 10 percent. If the difference in shrinkage exceeds thislevel, the resultant foam products will normally show an undesirablylarge variation in their dimensions.

EXAMPLE 1 AND REFERENCE EXPERIMENT 1

    ______________________________________                                        Base resin:                                                                            Ethylene-vinyl acetate copolymer (containing                                  10% of vinyl acetate, with density of 0.93                                    g/cm.sup.3 and MI of 1.5; EVATATE® D-2021                                 produced                                                                      by Sumitomo Chemical Co., Ltd.)                                                                   100    parts                                     Nucleators:                                                                            Calcium stearate    0.1    part                                               Calcium silicate    0.6    part                                      ______________________________________                                    

The foregoing resin and nucleators were fed, together with each additiveshown in Table 1, into a single-screw extruder of 40 mm in barreldiameter provided with a die having a round aperature of 5 mm indiameter. The resultant resin composition was melted and kneaded thereinwith each volatile blowing agent fed separately thereto (also shown inTable 1), and the resultant molten mixture was extruded through the dieinto the atmosphere to be expanded into a foam.

In Table 1, the additives A, B, C and D are N-stearyl acetamide,N-stearyl caprylamide, N-polyoxyethylene derivative of N-stearylacetamide (n=4) and N-decyl formamide, respectively. The blowing agentsI and II were dichlorodifluoromethane and 1,2-dichlorotetrafluoroethane,respectively.

In the formulations of the same procedure and conditions as those usedin Example 1 were repeated except that either no additive was used oradditives A', A" or C' were employed in place of additives A, B, C or Dof the Example 1 formulation; the additives A', A" and C' being N-octylacetamide, N-polyoxyethylene derivative of N-stearyl acetamide (n=25),and N-octyl octylamide, respectively. Experimental results of Example 1and Reference 1 are summarized in Table 1.

In all experiments of Reference 1, the resultant foams showed aconsiderable post-expansion shrinkage much of which remained even afterthey were left to stand over a long period of time. Also, they had aninferior quality with their surfaces replete with wrinkles andirregularities.

FIGS. 1 and 2 show changes in the volume and weight of the foamspecimens of Example 1, formulation 1, and Reference 1, formulation 1,as a function of time after expansion. As clearly seen from FIG. 1,excellent foams having a very little volumetric change with time can beproduced by extruding the resin compositions according to the presentinvention. Also, as evident from FIG. 2 showing the changes in theweight of the foam specimens of Example 1 Formulation 1, and Reference 1Formulation 1 as a function of time, the foams of Example 1 undergo afar less change in weight, implying that transmission of the foamingagent gas through cell walls is substantially suppressed in the foamsexpanded from the resin compositions according to the present invention.Further, such low volumetric and weight changes, as a result of reducedgas transmission through the cell walls, contributes to an improvementin the surface appearance of the resultant foams.

                                      TABLE 1                                     __________________________________________________________________________                                     State of di-                                                        Shrinkage of Foam                                                                       mensionally                                               Blowing   Vol. %    stabilized                                           Additive                                                                           Agent                                                                              Foam     10    foam (30                                             (Wt. (Wt. Density  Days After                                                                          days after                                           Parts)                                                                             Parts)                                                                             (g/cm.sup.3)                                                                       Max.                                                                              Expansion                                                                           expansion)                                   __________________________________________________________________________    Example 1                                                                     Formulation 1                                                                         A 1.0                                                                              I 21 0.035                                                                              14.7                                                                              11.2  G                                            2       A 1.0                                                                              II 28                                                                              0.034                                                                              12.9                                                                              10.5  G                                            3       B 1.0                                                                              I 21 0.035                                                                              14.7                                                                              11.0  G                                            4       B 1.0                                                                              II 28                                                                              0.034                                                                              12.8                                                                              10.3  G                                            5       C 1.0                                                                              I 21 0.033                                                                              14.6                                                                              10.7  G                                            6       D 1.0                                                                              I 21 0.033                                                                              14.5                                                                              10.6  G                                            Reference 1                                                                   Formulation 1*                                                                        None I 21 0.040                                                                              64.6                                                                              51.2  P                                            2*      None II 28                                                                              0.030                                                                              33.4                                                                              25.7  P                                            3*      A' 1.0                                                                             I 21 0.037                                                                              65.0                                                                              45.5  P                                            4*      A" 1.0                                                                             I 21 0.036                                                                              64.0                                                                              50.0  P                                            5*      C' 1.0                                                                             I 21 0.038                                                                              65.0                                                                              47.5  P                                            __________________________________________________________________________     *Not an example of the invention.                                        

EXAMPLE 2 AND REFERENCE 2

Example 1 and Reference 1 were repeated except that 100 parts ofpolyethylene (with density of 0.919 g/cm³ and MI of 2.0; F-1920®produced by Asahi-Dow Limited) was used as the base resin in place ofthe ethylene-vinylacetate copolymer of Example 1 and Reference 1 andexcept that the nucleator contents, namely calcium stearate and calciumsilicate contents were changed to 0.06 part and 0.35 part, respectively.The experimental results are summarized in Table 2, in which blowingagents I and III are dichlorodifluoromethane and butane.

                                      TABLE 2                                     __________________________________________________________________________                                     State of di-                                                        Shrinkage of Foam                                                                       mensionally                                               Blowing   Vol. %    stabilized                                           Additive                                                                           Agent                                                                              Foam     10    foam (30                                             (Wt. (Wt. Density  Days After                                                                          days after                                           Parts)                                                                             Parts)                                                                             (g/cm.sup.3)                                                                       Max.                                                                              Expansion                                                                           expansion)                                   __________________________________________________________________________    Example 2                                                                     Formulation 1                                                                         A 0.7                                                                              I 20 0.031                                                                              8.0 5.0   G                                            2       A 0.7                                                                              III 20                                                                             0.031                                                                              14.8                                                                              15.5  G                                            3       B 0.7                                                                              I 20 0.030                                                                              10.0                                                                              7.5   G                                            4       B 0.7                                                                              III 20                                                                             0.031                                                                              14.7                                                                              15.4  G                                            5       C 0.7                                                                              I 20 0.030                                                                              8.5 10.5  F                                            6       D 0.7                                                                              I 20 0.031                                                                              20.0                                                                              15.5  F                                            7       A 0.1                                                                              I 20 0.033                                                                              30.0                                                                              15.0  F                                            8*      A 0.05                                                                             I 20 0.031                                                                              40.0                                                                              25.0  P                                            Reference 2                                                                   Formulation 1*                                                                        None I 20 0.035                                                                              53.4                                                                              30.0  P                                            2*      None III 20                                                                             0.033                                                                              61.2                                                                              48.5  P                                            3*      A' 1.0                                                                             I 20 0.034                                                                              59.2                                                                              32.4  P                                            4*      A" 1.0                                                                             I 20 0.035                                                                              60.0                                                                              30.5  P                                            5*      C' 1.0                                                                             I 20 0.034                                                                              38.2                                                                              52.0  P                                            __________________________________________________________________________     *Not an example of the invention.                                        

EXAMPLE 3

The same polyethylene resin as that used in Example 2 was mixed andkneaded with 0.3 percent dicumyl peroxide as a crosslinking agent and1.5 percent of N-stearyl acetamide as an anti-shrinking agent to obtaina crosslinked polyethylene resin in the form of beads with an averagediameter of about 1.7 mm, having a gel ratio of about 60 percent. Thebeads were impregnated, in a pressure vessel, withdichlorodifluoromethane under increased pressure and heat and thencooled to obtain expandable crosslinked polyethylene beads containing 14percent of dichlorodifluoromethane. Heating with 0.23 kg/cm² G (22.6kPa) steam for 45 sec. gave primary expanded beads with a substantiallyuniform density distribution of about 90 kg/m³. Thereafter, the primaryexpanded beads were heated in a pressure vessel at 80° C. for 15 hourswhile being pressurized by compressed air of 10 kg/cm² G (981 kPa).Then, steam of 0.32 kg/cm² G (31.4 kPa) was blown over the beads to givesecondary expanded beads which had a density of 25 kg/m³.

The resultant secondary expanded beads, as pressurized by compressed airof 1.5 kg/cm² G (147 kPa), were charged in a mold of a press moldingmaching (Model ECHO-120 manufactured by Toyo Machinery & Metal Co.,Ltd.), and the mold was heated by steam of 1.2 kg/cm² (118 kPa) toobtain a molded article. The molded article had a density of 31 kg/m³and showed an improved fusion between beads.

The water absorption ratio of the resulting molded foam article was thendetermined by accurately measuring the volume (V) and weight (W_(o)) ofa 100 mm×100 mm×25 mm specimen of such article, by submerging suchspecimen in water (the top of the specimen being about 5 cm below thewater's surface) contained in a vacuum chamger and by reducing theinternal pressure of the chamber to 460 mm of mercury (absolute) andleaving the specimen to stand in the water under that condition for 10minutes. Thereafter, the internal pressure of the chamber was restoredto atmospheric and the specimen was removed therefrom and immersed inmethanol having a purity of 95% or more for 2 seconds. After air drying,the thus treated specimen for 5 minutes at 60° C., its weight (W₁) wasagain accurately measured and the measurements W_(o), W₁ and V wereapplied to the following formula: ##EQU2##

The water absorption value thus obtained provides an indication of theclosed cellular characteristics of the foam specimen tested, with lowwater absorption ratios (i.e., relatively small amounts of water beingabsorbed by the sample) indicating a relatively high degree of closedcell character for the foam sample tested.

In this particular example the water absorption ratio of the indicatedmolded foam article was less than 0.01 gram/cm³.

REFERENCE 3

Except that the additive N-stearyl acetamide was not used, the sameprocedure and conditions as those used in Example 3 were repeated toobtain the primary and secondary expanded beads, which had densities of110 kg/m³ and 30 kg/m³, respectively. These primary and secondaryexpanded beads exhibited substantial shrinkage and were replete withsurface wrinkles, as compared with those obtained in Example 3.

In addition, expandable beads of Reference 3, after being impregnatedwith the blowing agent, had a useful life of only 20 minutes which wassignificantly shorter than that observed in Example 3 (6 hours).

Further, the molded articles obtained in Reference 3 showed a higherwater absorption ratio (0.015 gm/cm³) as compared with Example 3, andits shock-absorbing properties and mechanical properties weresignificantly inferior to those achieved in Example 3.

What is claimed is:
 1. A foamable synthetic resin composition comprisinga normally solid olefin polymer, a volatile organic blowing agent andfrom 0.1 to 10 percent by weight, based on said olefin polymer of atleast one compound of formula I: ##STR3## wherein R₁ is an aliphatichydrocarbon group having 10 to 24 carbon atoms; R₂ is a hydrogen atom,an aliphatic hydrocarbon group having 1 to 24 carbon atoms, oroxyalkylene or polyoxyalkylene group represented by --R₄ --O--_(n) Awherein R₄ is a residual group of an aliphatic polyhydric alcohol having1 to 5 carbon atoms, A is a hydrogen atom or a residual group of analiphatic carboxylic acid having 1 to 24 carbon atoms and n is aninteger ranging from 1 to 20; and R₃ is a hydrogen atom or an aliphatichydrocarbon group having 1 to 7 carbon atoms.
 2. The foamable syntheticresin composition according to claim 1, wherein the aliphatic olefinpolymer comprises polyethylene or an ethylene copolymer composed of amajor proportion of ethylene and a minor proportion of one or more othermonomers compolymerizable therewith.
 3. The foamable synthetic resincomposition of claim 1 wherein the compound of the formula I isN-stearyl acetamide.
 4. The foamable synthetic resin composition ofclaim 1, wherein the compound of the formula I is N-stearyl caprylamide.5. A substantially closed-cell foam article composed predominantly of analiphatic olefin polymer composition having incorporated therein from0.1 to 10 weight percent based upon such olefin polymer of a compound ofthe formula I: ##STR4## wherein R₁ is an aliphatic hydrocarbon grouphaving 10 to 24 carbon atoms; R₂ is a hydrogen atom, an aliphatichydrocarbon group having 1 to 24 carbon atoms, or oxyalkylene orpolyoxyalkylene group represented by --R₄ --O--_(n) A wherein R₄ is aresidual group of an aliphatic polyhydric alcohol having 1 to 5 carbonatoms, A is a hydrogen atom or a residual group of an aliphaticcarboxylic acid having 1 to 24 carbon atoms and n is an integer rangingfrom 1 to 20; and R₃ is a hydrogen atom or an aliphatic hydrocarbongroup having 1 to 7 carbon atoms.
 6. The foam article of claim 1,wherein the aliphatic olefin polymer is polyethylene or an ethylenecopolymer derived from a major proportion of ethylene with a minorproportion of one or more other monomers copolymerizable therewith.